End user terminals for wireless communication systems (including radio and television) and portable cellular phones should be small, lightweight and inexpensive and have low power consumption.
The first generation of cellular telephone systems relied on analogue frequency modulation for speech transmission, and several standards have been developed, such as NMT450, NMT900, AMPS, and ETACS. The second generation of cellular systems is based on three different standards: in Europe and some countries in Asia and Australia—Global System For Mobile Communications (GSM), in north America—American Digital Cellular (ADC), and in Japan—Pacific Digital Cellular (PDC). These second generation systems all employ digital transmission for voice and data, including some digital services such as facsimile transmission and short messages. To make the portable terminals smaller and less expensive they make extensive use of integrated circuits. Most user terminals for 1st and 2nd nd generation systems are simple telephone terminals operating with a single telephone system and with limited data processing capability, such as personal data assistants (PDA's—diary, reminder, notebook) and simple games.
A wireless telephone user terminal (or handset) for GSM is described in U.S. Pat. No. 5,493,700, assigned to the assignee of the present invention. This user terminal includes a reference frequency generator comprising a crystal resonator supplying a fractional-N phase-locked loop frequency synthesizer, the frequency being corrected by synchronisation relative to a frequency of the received wireless communication signal.
Communication systems are now being prepared according to a third generation of standards. Among 3rd generation cellular standards are the UMTS 3GPP (3rd generation Partnership Project) and 3GPP2 standards,of the European Telecommunications Institute (‘ETSI’), the International Mobile Telecommunications-2000 (‘IMT-2000’) standards. It is desirable for the 3rd generation user terminals to be capable of functioning on at least the local 2nd generation standards as well as the 3rd generation standards, especially during the period of introduction of the 3rd generation and until its coverage is as extensive as the 2nd generation. However, the radio frequency (‘RF’) signals for the two generations are different and are not simple integer multiples or sub-multiples of each other.
In addition, the user terminals for 3rd generation wireless telephony are inherently capable of adaptation to function with accessories (headsets, printers, or local area networks, for example) and download of media (music, speech and video over the Internet) by short-range wireless communication with the co-operating devices. The personal area network standards, such as BlueTooth, the digital-to-analogue and analogue-to-digital converters in the audio channels involved, the inclusion of powerful microprocessors and the addition of location aware services requiring wireless communication with triangulation sources, such as the Global Positioning System (‘GPS’), increase very significantly the number of different, high precision radio frequency clock signals that must be generated and, especially at radio frequencies, the generation of sufficiently accurate and precise clock signals tends to be expensive and to have high power consumption levels.
Moreover, the frequency synthesizers used in the wireless transmitter and receiver stages of the different wireless communication applications often need to be synchronised separately relative to the respective received signals by automatic frequency control and the clock signals used for internal signal processing and data processing need to be controlled relative to the frequencies used in the transmitter and receiver stages. In particular, the different radio communication units in the terminal, such as GSM and WBCDMA and Bluetooth and GPS, for example need to be synchronized separately relative to the respective types of base stations, that is to say the GSM Base station, WBCDMA base station, Bluetooth master unit and GPS satellite master unit, in these examples, which are not synchronized between themselves. It is possible, but undesirable for several reasons, to provide separate crystal frequency references for the respective radio communication units, each crystal using its own automatic frequency correction to synchronize to the respective system networks separately.
The present invention provides an effective relatively low energy-consumption means of providing multiple clock frequencies. The invention is applicable to wireless telephony and also to other apparatus where multiple clock frequencies are required.